Rated 5 out of 5 stars

To get it working on firefox 4 b8 pre (or earlier) follow these instructions:
1 - download the addon rather than install (right click and select save as).
2 - unzip the contents to a temporary folder.
3 - open file install.rdf with notepad
4 - replace the contents with the text at the bottom of this post
5 - save the file
6 - rezip the contents
7 - go to firefox and select to install from file (change the browse contents to all files) and select the zip file.
8 - follow the normal install method and it should install as per usual. The GB dictionary option should appear with the US one in the usual way.

The adjustments in code were minimal. Just the version numbers for firefox/minefield and a tweak in the unpack code. I cant edit this add-on as its not mine.



<?xml version="1.0"?>

<RDF xmlns="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:em="http://www.mozilla.org/2004/em-rdf#">
<Description about="urn:mozilla:install-manifest">
<em:id>en-GB@dictionaries.addons.mozilla.org</em:id>
<em:unpack>true</em:unpack>
<em:version>1.19a</em:version>
<em:creator>Mark Tyndall</em:creator>
<em:contributor>various</em:contributor>
<em:homepageURL>https://addons.mozilla.org/en-GB/firefox/addon/3366</em:homepageURL>
<em:targetApplication>
<Description>
<em:id>{ec8030f7-c20a-464f-9b0e-13a3a9e97384}</em:id>
<em:minVersion>2.0</em:minVersion>
<em:maxVersion>4.0b8pre</em:maxVersion>
</Description>
</em:targetApplication>
<em:targetApplication>
<Description>
<em:id>{3550f703-e582-4d05-9a08-453d09bdfdc6}</em:id>
<em:minVersion>2.0</em:minVersion>
<em:maxVersion>3.3a1pre</em:maxVersion>
</Description>
</em:targetApplication>
<em:targetApplication>
<Description>
<em:id>{92650c4d-4b8e-4d2a-b7eb-24ecf4f6b63a}</em:id>
<em:minVersion>1.1a</em:minVersion>
<em:maxVersion>2.1b1</em:maxVersion>
</Description>
</em:targetApplication>
<em:name>United Kingdom English Spellchecker</em:name>
<em:description>English Great Britain (en-GB) Spellchecker</em:description>
</Description>
</RDF>

This review is for a previous version of the add-on (1.19).  This user has a previous review of this add-on.